1. Field of the Invention
This invention relates to the field of optical equipment, and more particularly to devices which test the surfaces of optical detectors for contamination that may degrade performance of the detector.
2. Description of the Prior Art
Many different kinds of systems utilize detectors of electromagnetic energy, especially within certain ranges of the electromagnetic spectrum. Detectors that are sensitive to visible light or infrared energy are particularly useful. As an example, a system that senses fires in military vehicles employs two detectors, one capable of detecting energy in the visible spectrum and the other capable of detecting energy in the infrared spectrum. These fire sensor detectors are typically mounted side-by-side in relatively inaccessible locations where dirt, grease, oil, and other contaminants can build up on the detector surfaces. Such contaminants on the detector surfaces reduce the amount of electromagnetic energy reaching the detectors, thereby degrading sensor performance. (In technical literature, the words "detector" and "sensor" are sometimes used synonymously. As used herein, the word "detector" refers to a radiation sensitive element that converts electromagnetic radiation to electrical signals. The word "sensor" refers to a system using at least one "detector", and which includes some other electronics to amplify or process the "detector" signals.)
Built-in test equipment has been used in the past to test for the build-up of contaminants on the face of detector or sensor systems. One such system in the prior art utilizes a protective window having a source of ultraviolet (UV) energy and a detector of ultraviolet energy both located behind the window. Ultraviolet energy is transmitted through the window and reflected back to the ultraviolet detector by a small reflector mounted just outside and to the side of the window.
Another such system in the prior art that tests itself for contaminant build-up utilizes visible spectrum only detectors to perform its sensing function. To perform its self-test, a light bulb mounted outside in front of the detectors is energized to stimulate each of the detectors with an appropriate signal.
In general, when more than one detector is used, such as in these prior art systems, each detector must be equipped with its own built-in test system to test its window for contamination. A possible exception is where the built-in test source--like a light bulb--is so large that it covers several detectors. In most systems, however, it is very undesirable for the test source to be so large since weight and size must be minimized.
While techniques are available to test UV and visible detectors for window contamination, such techniques are not available for infrared detectors. Infrared detectors, per se, often exhibit a broad spectral passband, but they are usually used with a narrow band filter to limit sensitivity to a region of interest. Suitable sources that could be mounted externally thus become impossible to find and automatic monitoring of contamination build-up is no longer feasible. Examination of optical surfaces thus becomes a task of periodic and frequent maintenance.
Since suitable external sources of IR detectors have not been available, internal sources are commonly used. These internal calibration sources (ICS's) are generally placed on the detector side of the filter and often internal to the detector. Using an ICS, no attempt is made to monitor window contamination; its role is solely for the functional testing of the detector and associated electronics.
Previous attempts to overcome this dilemma have generally focused on the test source: finding one that is suitable. Gas and dye lasers are too large and expensive. Lead salt diode lasers are small and compact, but require cryogenic cooling even to generate very small power outputs. Probably the best approach to this problem to date involves using a coil of wire behind an infrared-transmitting window. Even with this approach, however, the source must be moved very close to the detector windows and then pulsed with several amps of current. Thus, compact, directional, low power sources have not been available in the infrared region as they are in the visible and UV regions.
Additionally, it would be desirable if each detector or optical element did not have to be equipped with its own built-in external test source. The more external optical surfaces present, the more likely damage may be sustained in hostile environments. Further, cost and complexity increase very rapidly with the number of external elements present, whereas these factors can be minimized if internal elements can be substituted for external elements.